Electron emitter structure for field emission display

ABSTRACT

Electron emitter structure for field emission display, said display comprising a tabular vacuum chamber confined between a rigid transparent front plate and a substantially flat electron emitting structure comprising a plurality of emitting elements, the residual contaminant gas molecules being removed by transversal pumping through a plurality of pores spread out on said electron emitting structure in order to reach a layer of getter material uniformly distributed over the display area. The emitting elements may be provided by Spindt emitters, sharp or serrated metallic edges or carbon nanotubes. The electron emitting structure comprises an upper and a lower metallic layers plated over the upper and lower surfaces of an insulating plate, the latter consisting of a photo-etchable or plasma-etchable material, such as polyimide or SU8.

[0001] The present invention is related to flat panel informationdisplays and, more specifically, to FED (Field Emission Display) devicesbased on electron emission from sharp conducting objects.

[0002] It is already known that strong electric fields, in the order ofmegavolts per centimeter, can be used to provide cold cathode emissionfrom conducting surfaces. It is also well known that when the emittersare shaped as sharp needles or edges, the emission voltage can bereduced to more practical levels, such as a few kilovolts percentimeter.

[0003] Such effect can be efficiently used for fashioning electronicdevices which operate like electronic valves or, better, as cathode raytubes, with the advantage of eliminating the cathode heating and savingthe power needed when compared to the latter, thus increasing theoverall efficiency. Such a device is described in U.S. Pat. No.3,789,471 which shows structures that function as diodes and triodes,where the cathode is shaped like a sharp tip in which the concentrationof the electric field produces cold cathode electron emission. Themanufacturing process for such electrodes was first described by Spindtin 1968, hence those electrodes are known as Spindt emitters.¹

[0004] As shown in the above mentioned patent document, as well as inU.S. Pat. No. 3,665,241, the electron source comprises a plurality ofSpindt emnitters, conical or pyramidal, placed over a conductingsubstrate, with the addition of an accelerating grid-like structureconsisting of a conducting foil electrically insulated from thesubstrate, provided with holes having their centers coinciding with thetips of the Spindt emitters.

[0005] The drawings in FIG. 1 show several electron emitting structures:the perspective view in FIG. 1-a and the corresponding cross-sectionview in FIG. 1-b show an electron emitter structure consisting ofpyramidal Spindt emitters 12 where the holes 15 of the grid foil 14 aresquare shaped. Said pyramidal emitters are placed over a conductingsubstrate 11, insulated from the grid sheet by an insulating layer 13.The drawings in FIGS. 1-c and 1-d show cone-shaped Spindt emitters 12′,the holes 15′ in the grid 14′ being circular in this case. In bothembodiments shown in FIG. 1, the Spindt emitter tips are substantiallyon the same plane as the upper face of the grid foil 14 or 14′. Thedrawings also show that in the embodiments of FIGS. 1-a and 1-b, theconducting substrate is self-supported, while in the one shown in FIGS.1-c and 1-d, the conducting substrate rests upon an insulating base 10.

[0006] As described in the aforementioned documents, electrons areemitted when a negative voltage is applied to the substrate 11, the gridfoil voltage being positive. The amount of emitted electrons can becontrolled by varying the voltage applied to the grid 14 or 14′. Theaddition of a separating insulator plate 17 and an anode 16, as shown inFIG. 1-e, yields a triode-like structure. A positive voltage, higherthan the grid voltage, is applied to this anode. The assembled partsform a gas-tight chamber 18 which is evacuated.

[0007] This basic structure can be used for fashioning lighted panels,in which a transparent anode is coated with a layer of luminescentmaterial—“phosphor”—which emits light when struck by electrons,similarly to what happens in a CRT face.

[0008] A problem which occurs with devices of this kind lies in thecontamination of the vacuum by gas molecules which are graduallyreleased from the material surfaces. Experimental data show that suchdevices only operate reliably when the gas pressure inside the evacuatedchamber is equal or less than 10⁻⁶ torr. With higher pressures, the gasmolecules may become ionized; these ions are attracted by and strike theelectrically biased surfaces, impairing the emitting structures.Moreover, even when this ionization is absent, gas molecules areadsorbed by the exposed surfaces, modiifying the work function of theemitter material and degrading the phosphor layer.

[0009] The removal of the molecules from the region in which theelectrons travel is achieved by placing inside the evacuated chamber agetter which binds the contaminant gas molecules.

[0010]FIG. 2 shows a light emitting display built according to the knowntechnique. Said display comprises a cathode structure composed of aconducting backplate 21 that can be self-suporting or bonded to a rigidinsulating slab 20, this backplate being provided with a plurality ofSpindt emitters 22 centered at the bottom of through-holes 23 providedin an insulating panel 24 attached to the internal surface of saidconducting backplate, the outside surface of said insultaing panel beingoverlaid with a control grid 25 consisting of a conducting foil providedwith holes 26 concentric with said through-holes 23 and said emitters22, the assemblage of the above mentioned elements forming the electronemitting structure or the cathode structure. The display also comprisesa rigid transparent front plate 27, usually made of glass, having itsinternal surface coated with a transparent conducting film 28 (anode);the inside surface of this anode is overlaid with phosphor 29, either asa continuous layer or as a plurality of discrete spots which constitutethe picture elements—pixels.

[0011] The display shown in FIG. 2 differs from the assembly of FIG. 1-eby the fact that the vacuum chamber comprises the full extension 32 ofthe device, to allow the displacement, by gaseous diffusion, of thecontaminant molecules, from any place in the vacuum chamber to thegetter 33 which is placed on a trough 34 provided along one side of thedisplay. This displacement of the gas molecules along the length of thedisplay is called “longitudinal pumping”. The spacing elements betweenthe front plate and the cathode structure in the display of FIG. 2 havebeen omitted in this drawing for clearness sake.

[0012] The pixel definition, specially in the case of colored displays,hinges on the production of sharply defined electron beams, because thedefocussing of the beam will result in that a part of the electrons willimpinge on phosphor spots of different colors than intended. One of themain causes of this defocussing is the distance travelled by theelectrons between the tip of the Spindt emitter and the picture element,i.e., the phosphor spot. In displays built according to knowntechniques, this distance 35 is about one millimeter, resulting in anunacceptable image quality unless complex and expensive additionalstructures—not shown in the figure —are used to control the scatteringof the electron beams. A more straightforward way of lessening saidscattering would be to reduce the distance between the emitter structureand the front plate.

[0013] However, this reduction will give rise to a pressure gradientalong the display's length, impairing the vacuum in the regions of thedisplay farther from the getter. This effect depends on the relationbetween the display size 32, typically of the order of 10, 20 or morecentimeters, and the free gap 35 between the cathode structure and thefront plate. An adequate longitudinal pumping will result only when saidgap is equal or greater than 1 millimeter. However, as mentioned before,such large distances require the addition of complex and expensivestructures, such as the one described in U.S. Pat. No. 6,013,974.

[0014] The approximation between the front plate and the cathodestructure constitutes a more straightforward solution for thedefocussing problem, due to the fact that the reduction of the pathtraversed by the electrons before impinging in the front plate willreduce the spot illuminated by the electron beam, which will impingeupon one picture element only, doing away with the need for additionalfocussing means. However, this nearness diminishes the vaccumconductance, hindering the displacement of contamninant gas moleculesalong the display length, resulting in a residual pressure gradient.This lack of uniformity in the vacuum quality will bring about thedeterioration of the emitter elements as well as of the phosphor, whichwill be more intense on the central part of the display, resulting in alack of picture uniformity.

[0015] In view of the preceding, therefore, it is the aim of the presentinvention to produce an uniform vacuum, giving rise to a uniform imagequality in the full extension of the display, without recourse tocomplex and expensive focussing structures.

[0016] The above mentioned aim is achieved by the invention by providinga substantially uniform distribution of the getter along the fillextension of the display, the path of the gas molecules toward thegetter being substantially perpendicular to the electron emitterstructure, said path comprising a plurality of pores uniformlydistributed along said structure.

[0017] According to another feature of the invention, the depth betweenthe pore front and back openings is smaller than their transversedimension.

[0018] According to another feature of the invention, the getter isplaced in a chamber occupying the full extension of the display, saidchamber being placed between the rear face of the electron emittingstructure and the back closing plate of the display.

[0019] According to yet another feature of the invention, the getter isplated over the inside surface of said closing plate.

[0020] According to a further feature of the invention, the pore edgesare metal plated and function as electron emitters.

[0021] The foregoing characteristics, as well as other aspects andadvantages of the invention will become more evident from thedescription of the following embodiments, shown as examples and not in alimiting sense, as depicted in the attached drawings where similarreference numbers identify similar parts.

[0022]FIG. 1 shows the underlying principles of known Spindt emitters.

[0023]FIG. 2 shows a FED display built according to known techniques.

[0024]FIG. 3 is a section view of a FED display built according to theinvention.

[0025]FIG. 4 shows a perspective view of the electron emitting structureaccording to the invention.

[0026]FIG. 5 shows a cathode structure of a FED display in which theelectrons are emitted by the pore edges.

[0027]FIG. 6 shows an alternative arrangement of the electron emittingstructure depicted in the previous figure, in which the pore edges arecoated with a DLC layer.

[0028]FIG. 7 shows a preferred embodiment for the pore in the shape of apolygon.

[0029]FIG. 8 shows a further embodiment of the electron emittingstructure.

[0030]FIG. 9 shows an alternative distribution of emitter elements andpores.

[0031]FIG. 10 shows further versions of the electron emitting structuresin which the electrons are emitted by carbon nanotubes.

[0032] The display built according to the invention, depicted in FIG.3-a, comprises a front plate similar to the one in the known displayshown in FIG. 2, however it differs from the latter as regards theelectron emitting structure 37, as well as the back chamber 36 thatspans the full extension of the display. This chamber is positionedbetween the back of said structure and the inside face of the closingplate 20′. Said structure consists of an insulating plate 24 overlaidwith metallic conducting layers in both upper and lower surfaces. Thematerial of said plate is polymer that can be engraved by photo-etchingor plasma etching process, such as polyimide or SU8.

[0033] As shown in the detailed view of FIG. 3-b, the insulating plateis provided with a plurality of through holes or pores 38, allowing thecontaminating gas molecules to pass freely from the vacuum chamber 31 tothe getter layer 33′ that coats the inside surface of the back closingplate 39. This molecular diffusion proceeds in a direction perpendicularto the plane of the display, as shown by the arrows 62, being called“transverse pumping”. It should be noted that, in the present case, thepath traversed by the gas molecules to reach the getter is much shorterthan in the case of longitudinal pumping, the vacuum conductance being,therefore, correspondingly larger. Moreover, with the arrangement shownin FIG. 3 the vacuum conductance is not affected by the gap 35 betweenthe cathode structure 37 and the front plate; therefore, this distancecan be reduced as required to avoid defocussing of the electron beamsdue to scattering.

[0034]FIG. 4 shows a perspective view of the display built according tothe invention, with the front plate removed. As depicted, the pores 38are interspersed with the Spindt emitters 22, the getter layer 33′ beingvisible through said pores.

[0035] Notwithstanding the fact that FIGS. 3 and 4 show pores and Spindtemitters in roughly the same quantities, this relation can be changed asneeded by circumstances. Although the drawings depict said elements asbeing about the same size, in practical devices the emitters aresubstantially smaller than the pores. Typical Spindt emitters measureabout 1 micrometer, while the pore diameters are on the order of tens ofmicrometers. Therefore, cathode structures such as shown in FIG. 9 canbe fashioned, in which each pore 38 corresponds to a group 22′comprising several Spindt emitters, without overstepping the bounds ofthe invention.

[0036] In a second embodiment of the inventive concept, depicted in FIG.5, the electrons are not emitted by Spindt elements but by the sharpedges 41 of the metallic plating that, besides covering the lowersurface of insulating plate 45, extends in the upper direction coveringthe pore walls 43 and reaching the upper surface of said insulatingplate. This embodiment has the favorable feature of increasing the sizeof the region of high electric field concentration, i.e., the regionfrom which the electrons are emitted. Indeed, said emission can takeplace along the full edge of the pore wall plating, while in a Spindtdevice the electrons issue only from the tip of the cone or pyramid. Toinsure that emission occurs along the whole perimeter of the pore edge,said edge can be serrated, so that a large number of sharp tips areavailable for electron emission.

[0037]FIG. 6 shows an arrangement similar to the one of the precedingdrawing, however in this case the emitting edge 41 is overlaid with amembrane 44 of DLC (diamond-like carbon). This layer, which in practiceranges between 5 and 50 nanometers thick, reduces the work function atthe metal surface, facilitating the electron emission from said poreedges.

[0038] The pore shape is not restricted to a circle, as shown in FIG. 4.Actually the pores can be shaped as ovals, polygons or slits, providedthe distance between the side walls is greater than the depth measuredbetween the upper and lower openings. A specially effective shape isthat of a polygon having alternately outward and inward angles, such as,for example, in the polygons that satisfy Jordan's theorem.

[0039] The drawings in FIG. 7 show a triode type electron emittingstructure, in which each emitting pore 51 has the shape of a 6-pointstar. As depicted, the pore lies substantially at the center of acircular depression 52 in the insulating plate 57. In case of irregularor asymetric pores, the depression will be proportionately shaped. Asshown in the detailed view of FIG. 7-b, said depression 52 lies betweenthe edge of the metal-clad upper surface 56 of the insulating plate 57and the electron emitting elements, which happen to be the star points53. As is the case with the structures shown in FIGS. 5 and 6, theconducting foil 54 which covers the insulating plate lower face extendsupwards into the pore 51 side walls and reaches the bottom plane of saiddepression 52. Said foil may consist of a metal such as copper,molybdenium, tungsten, etc. The electron emission can be facilitated byplating the emitter points 54 with DLC or with a low-work-functionmaterial, such as a boron compound.

[0040] As shown in FIG. 7-b, the lower conducting foil 54 is connectedto the (−) pole of a power supply. The upper foil 56, which functions asthe control grid, is connected to the (+) pole of the same power supply,the emitting elements being negatively biased relative to the grid. Thebrightness of the light emitted by the phosphor layer 58 depends on theelectron kinetic energy, which is a function of the accelerating voltageapplied to the transparent conducting anode 59 overlying the internalface of the front plate 57. This accelerating voltage (++) is equal orgreater than 3 kV, which is much higher than the control grid voltage,typically 100 volts.

[0041] In a triode arrangement such as the one shown in FIG. 7, thevoltage between the electron emitting element and the control grid maybe varied with the purpose of controlling the electron beam intensityand thus the brightness of the illuminated spot. This control is madepossible by the fact that the distance between the emitter tip 53 andthe edge of the grid layer 56 is much smaller than the distance betweenthe electron emitting structure and the transparent conducting layer59—the anode—which overlays the front plate 57. Typical values are 2micrometers for the first distance and 300 micrometers for the second,wherefore the electrons emitted by the tips 53 travel along aparabola-like path 55, starting toward the grid and gradually veeringtoward the anode due to its stronger electric field.

[0042] In all the embodiments of the invention, the electron emission isstabilised by placing a resistance in series whith each electronemitting element. Said resistances are omitted in the drawings forclarity's sake.

[0043] In another embodiment of the invention, the back chamber 36 iseliminated by placing the back closing plate 20′ flush against the rearface of the electron emitting structure, such as depicted in FIG. 8. Inthis case, the pore will be shaped as a shallow well, in which thebottom opening is closed by the rear continouous metallic layer 21′sandwiched between the insulating plate 24 and said back closing plate20′. Thus, as depicted in FIG. 8, the getter 33″ will overlie only theexposed portion of the metallic layer that closes the bottom opening ofpore 38′. It should be stressed that, while the embodiment shown in FIG.8 shows Spindt emitters, the same basic idea is suitable for embodimentsemploying pores with emitting edges such as—but not limited to—the onesdepicted in FIGS. 5, 6 and 7.

[0044] In a second set of alternative—embodiments of the invention, theSpindt emitters are substituted by clumps of carbon nanotubes, whichalso emit electrons at room temperature. The embodiments employingcarbon nanotubes are shown in FIGS. 10-a, 10-b and 10-c. The first twoof these are equivalent to the electron emitting structures of FIGS. 3and 8, while the embodiment depicted in FIG. 10-c differs from theprevious ones by having a self-supporting insulating backing plate 64under the rear continuous metallic layer 21′ over which are placed thecarbon nanotube disk-like clumps 61. This latter structure can also beused with Spindt emitters. In all cases, the pores 38′ are interspersedwith the electron emitting elements, to provide a path that allow thecontaminating gas molecules to reach the getter layer 33′.

[0045] Said carbon nanotubes can also be used in conjunction with edgeemitting pores, in which case the nanotubes will be applied in a layerover the metallic edge bordering said pores.

[0046] As mentioned previously, each electron emitting element has aballast resistance connected in series for emission stabilisationpurposes, said electron emitting element being either a Spindt emitter,the pore metallic edge or a clump of carbon nanotubes. When emittingpores are used, said resistance can be provided by reducing thecross-section of the metallic plating on the pore walls.

[0047] Two kinds of biasing setups can be used in FED display deviceshaving electron emitting structures that use carbon nanotubes, such asthe ones shown in the drawings of FIG. 10. In the first, the carbonnanotubes are negatively biased, the grid layer 25 has a small positivebias and the anode is strongly positive. In the second biasing setup,the nanotubes are positive, the grid is negative and the anode isstrongly positive.

[0048] Additional advantages of the present invention will readily occurto those skilled in the art while keeping within the conceptual boundsof the invention. For instance, although the front plate is depicted asbeing coated on the inside with the transparent anode and the phosphoroverlaying said anode, the placement of these layers can be reversed, asis the usual practice in TV picture tubes. In such case, the phosphorlayer is applied directly over the inside surface of the front plate andthe anode consists of a thin reflecting aluinium film placed over thephosphor layer. This setup increases the image brightness and contrastdue to the reflection of the light emitted in the backward direction bysaid aluminum layer.

[0049] Therefore, in consideration of the preceding, the spirit andscope of the invention are limited and defined by the appended claims.

1. ELECTRON EMITTER STRUCTURE FOR FIELD EMISSION DISPLAY, said displaycomprising a tabular vacuum chamber (31) confined between a rigidtransparent front plate (27) internally overlaid with a transparentconducting anode (28) and a substantially flat electron emittingstructure (37) provided with electron-beam emitting means (22, 41, 53,61) and electron-beam current intensity control means (14, 14′, 25, 56),said structure comprising a substantially flat membrane-like insulatingsubstrate (24, 45) having its first surface facing said front plate, andits second surface—on the opposite side—facing a getter (33, 33′, 33″)distributed substantially uniformly over said display area, the passageof the residual contaminant gas molecules from said chamber to saidgetter being mediated by a multitude of pore-like apertures (38, 43, 51)distributed substantially uniformly over said insulating substrate (24,45), characterised by the fact that said insulating substrate's secondsurface, facing said getter, is overlaid with a first conducting foil(21, 42, 54) which is kept at a negative potential in relation to saidanode (28), said foil extending uninterrupted into at least some of saidpore-like apertures (43, 51) plating their side walls, the distal edges(41, 53) of said plating terminating substantially at the rims of saidpore-like apertures, the edges of said pore-like apertures not exceedingthe plane of said first surface.
 2. ELECTRON EMITTER STRUCTURE FOR FIELDEMISSION DISPLAY as claimed in claim 1, characterised by the fact thatsaid electron-beam current intensity control means are provided by asecond conducting foil (25, 56) overlaying said first surface of saidinsulating substrate (24, 45), said foil being provided with holesencompassing said pore-like apertures leaving an exposed extent ofinsulating substrate (52) between the rim of each pore-like aperture andthe edge of the foil, said foil being connected to a biasing voltagepower supply.
 3. ELECTRON EMITTER STRUCTURE FOR FIELD EMISSION DISPLAYas claimed in claim 2, characterised by the fact that the exposed extentof substrate left uncovered by said second conducting foil overlyingsaid first surface forms a shallow depression (52) bordering the rim ofsaid pore-like aperture, the edge of said second conducting foil (56)being terminated at the upper lip of said depression.
 4. ELECTRONEMITTER STRUCTURE FOR FIELD EMISSION DISPLAY as claimed in claim 1,characterised by the fact that said insulating substrate (24, 45) is apolymer engravable by photo-etching process.
 5. ELECTRON EMITTERSTRUCTURE FOR FIELD EMISSION DISPLAY as claimed in claim 1,characterised by the fact that said insulating substrate (24, 45) is apolymer engravable by plasma-etching process.
 6. ELECTRON EMITTERSTRUCTURE FOR FIELD EMISSION DISPLAY as claimed in claims 4 or 5,characterised by the fact that said polymer consists of polyimide. 7.ELECTRON EMITTER STRUCTURE FOR FIELD EMISSION DISPLAY as claimed inclaims 4 or 5, characterised by the fact that said polymer consists ofSU8.
 8. ELECTRON EMITTER STRUCTURE FOR FIELD EMISSION DISPLAY as claimedin any of the preceding claims, characterised by the fact that saidpore-like apertures (51) have their cross-section in the shape of apolygon that satisfies Jordan's theorem.
 9. ELECTRON EMITTER STRUCTUREFOR FIELD EMISSION DISPLAY as claimed in claim 8, characterised by thefact that said pore-like apertures (51) have their cross-section formedas a star-shaped polygon.
 10. ELECTRON EMITTER STRUCTURE FOR FIELDEMISSION DISPLAY as claimed in claims 1, 2, 3, 8 or 9 characterised bythe fact that said distal terminating edges (41, 53) of the side wallsplating of said pore-like apertures (43, 51) are overlaid withdiamond-like carbon—DLC (44).
 11. ELECTRON EMITTER STRUCTURE FOR FIELDEMISSION DISPLAY as claimed in claims 1, 2, 3, 8 or 9 characterised bythe fact that said distal terminating edges (41, 53) of the side wallsplating of said pore-like apertures (43, 51) are overlaid with alowwork-function material.
 12. ELECTRON EMITTER STRUCTURE FOR FIELDEMISSION DISPLAY as claimed in claim 11, characterised by the fact thatsaid low-work-function material is a boron compound.
 13. ELECTRONEMITTER STRUCTURE FOR FIELD EMISSION DISPLAY as claimed in claims 1, 2,3, 8 or 9 characterised by the fact that said distal terminating edges(41, 53) of the side walls plating of said pore-like apertures (43, 51)are overlaid with carbon nanotubes.
 14. ELECTRON EMITTER STRUCTURE FORFIELD EMISSION DISPLAY as claimed in any of the preceding claims,characterised by the fact that in said pore-like apertures the depthbetween the entrance and the exit openings is smaller than thetransverse dimension.
 15. ELECTRON EMITTER STRUCTURE FOR FIELD EMISSIONDISPLAY consisting of a membrane-like insulating substrate (21, 24, 45),with a first surface facing a tabular vacuum chamber (31) and saidsecond surface facing getter material (33, 33′) distributedsubstantially uniformly over an area equivalent to the area of saidelectron emitter structure, said getter material absorbing thecontaminant gas molecules contained in said tabular vacuum chamber,characterised by the fact that the travel path (62) between said vacuumchamber and said getter material comprises a first plurality ofpore-like apertures (38, 38′, 51) having the depth between the entranceand exit openings smaller than their transverse dimension and by thefact that said membrane-like insulating substrate is overlaid on bothfaces with conducting foils (21, 21′, 25, 54, 56).
 16. ELECTRON EMITTERSTRUCTURE FOR FIELD EMISSION DISPLAY as claimed in claim 15,characterised by the fact that said first plurality of pore-likeapertures are provided with electron emitting means consisting of thedistal terminating edges (53) of the side-walls' conductive platingwhich is electrically contiguous with the conducting foil (54) facingsaid getter material.
 17. ELECTRON EMITTER STRUCTURE FOR FIELD EMISSIONDISPLAY as claimed in claim 15, characterised by the fact that saidelectron emitter structure comprises additionally a second plurality ofpore-like apertures (23, 23) provided with electron emitting means. 18.ELECTRON EMITTER STRUCTURE FOR FIELD EMISSION DISPLAY as claimed inclaim 17, characterised by the fact that said electron-emitting meansare provided by the distal terminating edges (41, 53) of the conductiongplating of said pore-like apertures' side walls, said plating beingcontinuous with the conductive foil (42, 54) overlaying said secondsurface of said insulating substrate.
 19. ELECTRON EMITTER STRUCTURE FORFIELD EMISSION DISPLAY as claimed in claims 15 or 17, characterised bythe fact that said second plurality of pore-like apertures are shaped aswells (23, 23′) which have their bottoms closed by the unbrokenextension of the conductive foil (21, 21′) overlaying said substrate'ssecond surface, the electron emitting means (22, 61) being placed at thebottom of said wells in electrical contact with said conductive foil.20. ELECTRON EMITTER STRUCTURE FOR FIELD EMISSION DISPLAY as claimed inclaim 19, characterised by the fact that said electron emitting meansare provided by Spindt emitters (22).
 21. ELECTRON EMITTER STRUCTURE FORFIELD EMISSION DISPLAY as claimed in claim 19, characterised by the factthat said electron-emitting means are provided by clumps of nanotubes(61).
 22. ELECTRON EMITTING STRUCTURE FOR FIELD EMISSION DISPLAY asclaimed in any of the preceding claims, characterised by the fact that aballast resistance is provided in series with each electron-emittingelement.
 23. ELECTRON EMITTING STRUCTURE FOR FIELD EMISSION DISPLAY asclaimed in claim 22, characterised by the fact said ballast resistanceis provided by reducing the thickness of the conductive plating of theside walls of said pore-like apertures.